Interface Level

Parameters

Determines whether caller can sleep for memory. Possible flags are KM_SLEEP to allow sleeping until memory is available, or KM_NOSLEEP to return NULL immediately if memory is not available.

buf

Pointer to allocated memory.

Description

The kmem_alloc() function allocates size bytes of kernel memory and returns a
pointer to the allocated memory. The allocated memory is at least double-word
aligned, so it can hold any C data structure. No greater alignment
can be assumed. flag determines whether the caller can sleep for memory.
KM_SLEEP allocations may sleep but are guaranteed to succeed. KM_NOSLEEP allocations are
guaranteed not to sleep but may fail (return NULL) if no memory
is currently available. The initial contents of memory allocated using kmem_alloc() are
random garbage.

The kmem_zalloc() function is like kmem_alloc() but returns zero-filled memory.

The kmem_free() function frees previously allocated kernel memory. The buffer address and
size must exactly match the original allocation. Memory cannot be returned piecemeal.

Return Values

If successful, kmem_alloc() and kmem_zalloc() return a pointer to the allocated memory.
If KM_NOSLEEP is set and memory cannot be allocated without sleeping, kmem_alloc()
and kmem_zalloc() return NULL.

Context

The kmem_alloc() and kmem_zalloc() functions can be called from interrupt context only
if the KM_NOSLEEP flag is set. They can be called from user
context with any valid flag. The kmem_free() function can be called from
from user, interrupt, or kernel context.

See Also

Warnings

Memory allocated using kmem_alloc() is not paged. Available memory is therefore limited
by the total physical memory on the system. It is also limited
by the available kernel virtual address space, which is often the more
restrictive constraint on large-memory configurations.

Excessive use of kernel memory is likely to affect overall system performance.
Overcommitment of kernel memory will cause the system to hang or panic.

Misuse of the kernel memory allocator, such as writing past the end
of a buffer, using a buffer after freeing it, freeing a buffer
twice, or freeing a null or invalid pointer, will corrupt the kernel
heap and may cause the system to corrupt data or panic.

The initial contents of memory allocated using kmem_alloc() are random garbage. This
random garbage may include secure kernel data. Therefore, uninitialized kernel memory should
be handled carefully. For example, never copyout(9F) a potentially uninitialized buffer.